This invention relates to marine apparatus for supporting within its elastic limit elongate pipe or the like during underwater pipelaying operations in which the pipe is laid from a surface vessel onto a submerged surface.
Current trends of offshore oil and gas procurement indicate that the drilling and working of underwater mineral deposits will be increasingly important in providing the world's energy requirements. Pipelines are often laid underwater to connect offshore oil and gas wells with a production platform or with a gathering station which may be located on a platform some distance away. Also, pipelines may be utilized to connect offshore oil and gas wells to an onshore production center. In such a situation, the wellhead assembly is positioned on the ocean floor over the well and the production pipeline runs from the wellhead assembly along the ocean floor to the onshore facilities.
Pipelaying operations are generally conducted with a pipelaying barge on which a continuous pipeline is assembled by joining together, as by welding, several lengths of pipe until a pipeline the necessary length has been assembled. The lowering of such a pipeline into the water from an assembly barge under tension results in some bending of the pipe. The amount of bending that takes place in the pipe varies considerably and depends upon the weight, the diameter, wall thickness and coating thickness of the pipe, the material of the pipe, and the depth of the water. In performing the underwater pipelaying operation, it is important that the elastic limit of the pipe not be exceeded. If the radius of curvature of the pipe is too small, the pipe may become overstressed and permanently damaged. When a bend is too short, as may happen in very deep water, the elastic limit of the pipe might be exceeded so that the pipe becomes kinked and deformed.
In order to prevent damage to the pipe being laid, it has been necessary to resort to a partially submerged rigid ramp or stinger underlying the pipe in its path to the submerged surface to support the pipe in an acceptable profile to prevent damage. Originally, rigid support ramps were designed to operate in relatively shallow water with the lower end resting on the submerged surface upon which the pipeline was to be laid. In pipelaying operations in which deeper water was encountered, the ramp was found to be unsatisfactory due to stability problems. The stabilizing effect of pipe tension was found to be very helpful and improved the ramp technique of pipelaying. The application of tension to the launched pipe string prevented undue curvature thereof in the suspended portion between the barge and the submerged surface thereby limiting pipe stress.
As pipelaying operations moved into yet deeper water, longer rigid ramps were used but were found to be self destructive. The long rigid ramps were next cut into sections and hinged for articulation. The articulated ramp along with pipe string tension improved deep water pipelaying as buoyancy was given to the overbend portion of the profile tending to relieve stress on the pipe string. A typical arrangement of an articulated ramp in combination with pipe tension means is described in U.S. Pat. No. 3,321,925, issued to Clarence W. Shaw and assigned to J. Ray McDermott and Co., Inc., the assignor and assignee of this application. Articulated ramps have been pivotally connected to the lay barge. Such a pivoted, articulated ramp is that described in U.S. Pat. No. 3,390,532, issued to Joseph B. Lawrence. With the freedom to flex at the articulated ramp joints and at the pivotal connection to the barge, the articulated ramp configuration tends to be unstable in the water and subject to great movement. The permissibility of such flexure negates some of the advantage gained through the articulation of the ramp.
In pipelaying operations which are being conducted in deep water, which may be several hundred feet deep, a great length of pipeline is held suspended between the submerged surface and the lay barge. The suspended length of pipe can act as a pendulum, and small forces acting upon the suspended pipe can cause large oscillatory motions. Therefore, even small currents creating force against the suspended pipeline can produce a great torque at the sections of pipe near the lay barge. Such torque may, of course, effect a stress in the pipe which exceeds the elastic limit of the pipe.
The articulated ramps in use today have an additional drawback in that typically they must be assembled prior to delivery to the job site. The ordeal of transporting such a large, unseaworthy structure can be both tedious and expensive.
Another problem with articulated ramp structure is that the hydraulic jacks used to rigidify the ramp are subject to damage and consequent failure. This occurs because forces acting about a hinge connection, which include the upwardly acting forces exerted by the buoyancy of the adjacent support member and the downwardly acting forces exerted by the weight of the pipe, may get out of balance giving rise to a resultant force. The resultant force can be of such a great magnitude as to exert a torque about the hinge that exceeds the jack capacity. Jack failure may permit adjacent members to assume an unsuitable relative inclination causing damage to the adjacent section of pipe.